1. Field of the Invention
The invention is related to light weight aircraft. More particularly, the invention is related to a system and method for improving wing span efficiency and pitching control moments on lightweight aircraft.
2. Background Art
Sail wings have been proposed for light weight aircraft in the past, and are commonly used on hang gliders. One advantage of sail wings is that they allow a large amount of wing area to be folded up and stowed in a small volume.
However, the flexible membrane of the sail requires significant tension to control its shape, both in camber and twist. Even then, large twist angles can occur along the span, as the sail forms catenary type curves to convert its pressure load into membrane tension. To lower the camber and twist, the tension must be increased. This tension must be reacted by rigid structure, typically the “mast” or leading edge spar, as bending and compression loads. To reduce twist and/or camber, more tension is needed, which means more compression and bending in the spar, which increases the spar mass. Higher tension loads in the sail also require a thicker and heavier sail membrane.
The chordwise catenary curve results in airfoil camber. While this is good for production of lift, it also gives a large nose down pitching moment. The large nose-down pitching moment must either be reacted out by an aft center of gravity, which is unstable, or by a large horizontal tail.
The spanwise curve results in significant twist in the wing, which results in outer half of the wing being very poor at producing lift. Because the outer portion of the wing produces less lift than the inner portion of the wing, additional wing area must be provided to make up for the deficiency. Further still, the non-uniform distribution of lift means that the wing has a lift distribution that produces greater drag due to lift. More wing area means more weight, which can result in less usable load.
Thus, while sail wings are conceptually simple, they tend to have poor performance. Thus, a need exists for a wing configuration for a light weight aircraft that can reduce the induced drag and control the pitching moment.